The invention relates to the field of treatment for cardiac arrhythmias, and more particularly to methods and apparatus for delivering the optimum duration of an electrical shock to the heart during treatment of cardiac arrhythmias.
Ventricular fibrillation, an often fatal heart arrhythmia, can be terminated by the application of one or more electrical current pulses delivered to the heart through electrodes applied to the chest or implanted within the body. Since the first use on humans of a completely implantable cardiac defibrillator in 1980, research has focussed on making continually smaller and more efficient defibrillation devices. In addition, reducing the defibrillation threshold (DFT) energy level applied to the heart by the defibrillation pulses reduces the likelihood of damaging tissue adjacent the electrodes.
A conventional implantable defibrillator includes an electrical pulse generator and an arrhythmia detection circuit coupled to the heart by a series of two or more electrodes implanted in the body. A battery power supply, and one or more charge storage capacitors are used for delivering defibrillation shocks in the form of electrical current pulses to the heart.
Currently, the primary constraint in reducing the size of an implantable defibrillator is reducing the battery size and the size of the storage capacitor(s). Accordingly, improvements in the area of implantable defibrillators have focussed in two areas: (1) more efficient defibrillation waveforms, and (2) more efficient electrode configurations and placements. Stated in other words, the primary variables that can be adjusted in the design to lower the shock strength required for defibrillation include those variables relating to the defibrillation waveform, such as duration, polarity, and waveshape, and those variables relating to the electrodes, such as materials, size, shape, and location.
An example of a development in the area of electrodes is U.S. Pat. No. 4,827,932 to Ideker et al. which relates to a pair of spaced apart epicardial implantable defibrillation patch electrodes. A respective patch electrode is attached over each of the right and left ventricles in an attempt to achieve a uniform voltage gradient throughout the entire ventricular mass.
In the area of defibrillation waveforms, U.S. Pat. No. 4,641,656 to Smits discloses a method of applying a sequence of defibrillating pulses to the heart from a series of four electrodes. Two adjacent electrodes have positive polarity and the other two electrodes have negative polarity in an attempt to concentrate defibrillation energy in the heart wall rather than through the center of the heart. Two or more such pulses are applied, with a reverse in polarity of one pair of opposing electrodes between each pulse. Another pulsing scheme is disclosed wherein the polarity of the four electrodes alternates with each adjacent electrode, and with all four electrodes used simultaneously to defibrillate the heart.
Other examples of defibrillating waveforms are disclosed in U.S. Pat. No. 4,637,397 to Jones et al., U.S. Pat. No. 4,800,883 to Winstrom, and U.S. Pat. No. 4,821,723 to Baker, Jr. et al. These patents disclose multiphasic defibrillation waveforms wherein the polarity of pulses is reversed. U.S. Pat. No. 4,768,512 to Imran relates to a high frequency truncated exponential waveform. U.S. Pat. No. 4,727,877 to Kallok discloses a transvenous lead configuration wherein a first electrical pulse is delivered to a first pair of electrodes between the right ventricular apex and the superior vena cava, and after a predetermined delay, a second pulse is delivered to a second pair of electrodes between the right ventricular apex and the coronary sinus.
None of these efforts, however, sufficiently control the waveform to maximize the efficiency of the defibrillation pulses and thereby reduce the risk of damage to adjacent tissue and minimize the size of batteries, capacitors and other defibrillator hardware.
It is therefore, one object of the present invention to provide a method and apparatus for producing an optimum waveform for treating cardiac arrhythmias of a subject. It is a further object that the defibrillator provide an optimum monophasic or biphasic waveform for defibrillating the heart of a subject.
An additional object of the present invention is to provide an apparatus with optimal capacitance for producing waveforms which produce an electric counter-shock pulse for treating cardiac arrythmia of the heart of a subject and methods of selecting such an apparatus to thereby reduce the size of the battery and capacitor required for such an apparatus. The present object additionally facilitates implanting such an apparatus in a subject.
A further object of the present invention is to provide a waveform which minimizes damage to the cardiac tissue of the area of the heart receiving the counter-shock waveform signal.
These objects and advantages are achieved by a first embodiment of an apparatus according to the present invention. The first embodiment being a cardiac electric counter-shock apparatus, comprising circuitry for delivering a waveform signal to a pair of electrodes. The apparatus further comprises circuitry to detect an electrical characteristic of a pair of electrodes during delivery of the waveform signal and for calculating a waveform time constant (ts) from the detected electrical characteristic. The defibrillator stores a model time constant (tm) and calculates when a peak membrane voltage is reached based on the waveform time constant and the model time constant. Finally, the apparatus comprises switching circuitry to interrupt the waveform signal when the peak membrane voltage is reached.
A further aspect of the first embodiment of the present invention optionally provides a biphasic cardiac defibrillation apparatus having circuitry for delivering a second waveform after the first waveform where the second waveform has a polarity opposite that of the first waveform. This biphasic apparatus may also detect an electrical characteristic of a pair of electrodes during delivery of the second waveform signal and determine a waveform time constant (ts) from the electrical characteristic during the second waveform signal. The apparatus also calculates when the membrane baseline voltage is reached from the second waveform time constant and the model time constant and interrupts the second waveform signal when the membrane baseline voltage is reached.
A second embodiment of the present invention provides a cardiac arrythmia treatment method comprising delivering a waveform signal to a pair of electrodes when the electrodes are positioned for defibrillating the heart of a subject. An electrical characteristic of the pair of electrodes is detected during delivery of the truncated exponential waveform signal and a waveform time constant (ts) is determined from the electrical characteristic. A model time constant (tm) for a model response to the waveform is provided and the time when a peak membrane voltage is reached is determined from the waveform time constant and the model time constant. The waveform signal is then interrupted when the peak membrane voltage is reached.
A further aspect of the method comprises delivering a second truncated waveform signal of opposite polarity to the first waveform signal to the pair of electrodes. Optionally, an electrical characteristic of the pair of electrodes is detected during delivery of the second waveform signal and determines a waveform time constant (ts) from the electrical characteristic during the second waveform signal. The time when the membrane baseline voltage is reached is then calculated from the second waveform time constant and the model time constant. The second waveform signal is then interrupted when the membrane baseline voltage is reached.
A third embodiment of the present invention provides a method of selecting a cardiac arrythmia treatment apparatus for implantation in a subject. The selection method comprises providing a set of implantable cardiac arrythmia treatment apparatus which deliver a waveform signal, each member of the set having a different value of storage capacitor for delivering the waveform signal. A pair of electrodes are implanted in a subject, with the electrodes positioned for providing an electric counter-shock to the heart of the subject. The impedance across the pair of electrodes is measured after implantation. The cardiac arrythmia treatment apparatus is then selected from the set of apparatus based on the impedance and the storage capacitance of the apparatus. The selected apparatus is then implanted in the subject.
A fourth embodiment of the present invention is a set of implantable cardiac arrythmia treatment apparatus which deliver a waveform signal, each member of the set having a storage capacitor for delivering the waveform signal, and wherein each member of the set has a storage capacitor with a fixed capacitance different from the other members of the set.